The deadlock happened as the main thread wanted to join
SearchAndParseThread (while owning the solar mutex), but
SearchAndParseThread wanted to take the solar mutex to import a graphic.

Fix the deadlock by checking for the termination flag earlier, so we
don't take any new mutexes if termination was requested already.

Change-Id: Idddaadadc693610d9f31a14300b22aff8d452756
Reviewed-on: https://gerrit.libreoffice.org/55284Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>